When you closed the window with multiple tabs and left another one with only YouTube, you effectively told Firefox to save only the tab with YouTube, since that was the only session left when you exited Firefox. It saves only one session when closed.

Yes, I know, I would normally force quit FF so it backs up, but I forgot on this occassion. But in my searches I found plenty of people with similar problems but they were told on here to look for sessoinrestore-1js and they were able to restore previous sessions using that, but I cant find ANY sessionrestore js at all let alone one marked "1".

I ve found a few files I think may help, for instance I ve copied a file called "LastCrash" from 2 days ago, but I m unsure how I would use it to restore a session.

And some "upgrade.js" files from a month ago

I believe Firefox saves the session if more than one tab was open, not if there was only one tab.

Sort of, there are some backups in my FF folder, from months ago, apparently there should be more according to other threads but not in mine. Its bad design, what happens if you accidentally close tabs ? But I m kinda getting used to bad design in FF, it used to be good, but now its slow, dim witted rubbish.